In short, (A)SWM requires the necessary spigot files in your local Maven repository. The easiest way to do this is to use Spigot's build tools [Link]. You can find all the information you need in the Wiki.
Summary from the Wiki:
- Download and install Git [Link]
- Download and install Java 8 (AdoptOpenJDK works) [Link]
- Download the BuildTools.jar [Link]
- Open a terminal or shell (the GIT shell also works) and navigate to the file BuildTools.jar
- Build the following glasses, they will automatically be added to your local Maven repo
java -jar BuildTools.jar --rev 1.16.3
java -jar BuildTools.jar --rev 1.16.2
java -jar BuildTools.jar --rev 1.16.1
- Make changes to the (A)SWM and compile it into Maven using `package'.